test/power: add delay before checking CPU frequency

test/power: add delay before checking CPU frequency

For some platforms the newly-set frequency may not be effective
immediately. If we didn't get the right value from cpuinfo_cur_freq
immediately, add 10ms delay each time before rechecking until
timeout.

From our test, for some arm platforms, it requires up to 700ms when
going from a minimum to a maximum frequency. And it's not the
driver/software issue.

Fixes: ed7c51a6a680 ("app/test: vm power management")
Cc: stable@dpdk.org
Signed-off-by: Richael Zhuang <richael.zhuang@arm.com>
Reviewed-by: David Hunt <david.hunt@intel.com>